A characteristic feature of the copper oxide high-temperature superconductors is the dichotomy between the electronic excitations along the nodal (diagonal) and antinodal (parallel to the Cu-O bonds) directions in momentum space, generally assumed to be linked to the 'd-wave' symmetry of the superconducting state. Angle-resolved photoemission measurements in the superconducting state have revealed a quasiparticle spectrum with a d-wave gap structure that exhibits a maximum along the antinodal direction and vanishes along the nodal direction 1 . Subsequent measurements have shown that, at low doping levels, this gap structure persists even in the high-temperature metallic state, although the nodal points of the superconducting state spread out in finite 'Fermi arcs' 2 . This is the so-called pseudogap phase, and it has been assumed that it is closely linked to the superconducting state, either by assigning it to fluctuating superconductivity 3 or by invoking orders which are natural competitors of d-wave superconductors 4,5 . Here we report experimental evidence that a very similar pseudogap state with a nodal-antinodal dichotomous character exists in a system that is markedly different from a superconductor: the ferromagnetic metallic groundstate of the colossal magnetoresistive bilayer manganite La 1.2 Sr 1.8 Mn 2 O 7 . Our findings therefore cast doubt on the assumption that the pseudogap state in the copper oxides and the nodal-antinodal dichotomy are hallmarks of the superconductivity state. La 1.2 Sr 1.8 Mn 2 O 7 (LSMO) is a prototypical bilayer manganite that exhibits the colossal magnetoresistance (CMR) effect-the extremely large drop in resistivity induced by application of a magnetic field near the Curie temperature (T C ) 6,7 . The CMR effect exploits a metalinsulator transition between a low-temperature ferromagnetic-metallic ground state and a high-temperature paramagnetic-insulating phase. The nature of the ferromagnetic-metallic ground state in LSMO remains highly controversial. On the one hand, the underlying Fermi surface and band structure have clear resemblance to those of the copper oxides 8 (Lin, H., Sahrakorpi, S., Barbiellini, B. & Bansil, A., personal communication on full potential band structure and Fermi surface computations for x ¼ 0.4). On the other hand, previous angle-resolved photoemission (ARPES) investigations revealed no quasiparticle peak, a suppression of spectral weight at the Fermi level (E F ) (the pseudogap), and an unusually light effective mass, a factor of two lighter than the calculated band structure value. This last observation is puzzling given the general expectation of strong interactions in the manganites. Moreover, the value for the inplane conductivity calculated with the ARPES parameters is nearly one order of magnitude higher than that measured by transport 9-11 .Our ARPES experiments resolve the controversy of the lowtemperature ferromagnetic-metallic groundstate of LSMO by demonstrating that its electronic structure is strikingly similar to that found in th...

High temperature cuprate superconductivity remains a defining problem in condensed matter physics.Among myriad approaches to addressing this problem has been the study of alternative transition metal oxides with similar structures and 3d electron count that are suggested as proxies for cuprate physics.None of these analogs has been superconducting, and few are even metallic. Here, we report that the lowvalent, quasi-two-dimensional trilayer compound, Pr4Ni3O8 avoids a charge-stripe ordered phase previously reported for La4Ni3O8, leading to a metallic ground state. By combining x-ray absorption spectroscopy and density functional theory calculations, we further find that metallic Pr4Ni3O8 exhibits a low-spin configuration and significant orbital polarization of the unoccupied eg states with pronounced dx 2 -y 2 character near the Fermi energy, both hallmarks of the cuprate superconductors. Belonging to a regime of 3d electron count found for hole-doped cuprates, Pr4Ni3O8 thus represents one of the closest analogies to cuprates yet reported and a singularly promising candidate for high-Tc superconductivity if appropriately doped.

Perovskite oxides are an important class of oxygen evolution reaction (OER) catalysts in alkaline media, despite the elusive nature of their active sites. Here, we demonstrate that the origin of the OER activity in a La 1−x Sr x CoO 3 model perovskite arises from a thin surface layer of Co hydr(oxy)oxide (CoO x H y ) that interacts with trace-level Fe species present in the electrolyte, creating dynamically stable active sites. Generation of the hydr(oxy)oxide layer is a consequence of a surface evolution process driven by the A-site dissolution and O-vacancy creation. In turn, this imparts a 10-fold improvement in stability against Co dissolution and a 3-fold increase in the activity−stability factor for CoO x H y / LSCO when compared to nanoscale Co-hydr(oxy)oxides clusters. Our results suggest new design rules for active and stable perovskite oxide-based OER materials.

We present small-angle neutron scattering data proving that, on the insulating side of the metal-insulator transition, the doped perovskite cobaltite La(1-x)Sr(x)CoO(3) phase separates into ferromagnetic metallic clusters embedded in a nonferromagnetic matrix. This induces a hysteretic magnetoresistance, with temperature and field dependence characteristic of intergranular giant magnetoresistance (GMR). We argue that this system is a natural analog to the artificial structures fabricated by depositing nanoscale ferromagnetic particles in a metallic or insulating matrix; i.e., this material displays a GMR effect without the deliberate introduction of chemical interfaces.

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YBaCo4O7 belongs to a new class of geometrically frustrated magnets like the pyrochlores, in which Co-spins occupy corners of tetrahedra. The structure can be viewed as an alternating stacking of Kagomé and triangular layers. Exactly half of the triangular units of the Kagomé plane are capped by Co ions to form columns running perpendicular to the Kagomé sheets. Neutron powder diffraction reveals a broad temperature range of diffuse magnetic scattering, followed by long range magnetic ordering below 110K. A unique low-temperature magnetic structure simultaneously satisfies an S=0 arrangement in the uncapped triangular units and antiferromagnetic coupling along the columns. A spin reorientation above 30K tracks the relative strengths of the in-plane and out-of-plane interactions.PACS numbers: 25.40. Dn, 75.25.+z, Magnetic frustration has attracted considerable interest over the last 50 years [1,2,3]. Whether the frustration arises from competing exchange interactions or the peculiar geometry of the spin lattice, it invariably leads to unconventional magnetic states at low temperatures. Frustrated systems commonly exhibit the persistence of strong spin fluctuations at low temperatures and, at least, partial suppression of the magnetic order. Various spin states can be stabilized below the cooperative paramagnetic regime, such as spin liquid, spin glass or spin ice. In some cases, long range magnetic order is established, either by structural distortions that lift the ground state degeneracy, or by the "order-bydisorder" mechanism [4]. Kagomé antiferromagnets have been widely studied, in particular because a spin-liquid state is predicted at low temperatures for the S=1/2 Heisenberg system. However, real Kagomé lattices display either spin glass behavior or long range order with the well known propagation vectors k=0 and k=( 1 3 , 1 3 ) (often referred to as k=( √ 3, √ 3)) structures [5,6]. More exotic ground states, including incommensurate spin density waves and cycloidal structures, have been found in related lattices such as the so-called Kagome-staircase [7].Recently, a new class of geometrically frustrated magnets, with formula RBaCo 4 O 7 (R=Y,rare-earth ion) has been reported [8,9,10]. The crystal structure is built up of alternating Kagomé and triangular cobalt lattices, in a similar way as SrCr 9x Ga 12−9x O 19 (SCGO) [11], but with magnetic Co 2+ /Co 3+ ions in a 3:1 ratio on both crystallographic sites (S=1.625 on average). The magnetic network in RBaCo 4 O 7 is reminiscent of that of hexagonal ice 1h [12], but with one important difference: here, half of the triangles in the Kagomé sheets are bi-capped by Co atoms of the adjacent triangular layers, providing magnetic super-exchange interactions along the third direction. An added element of interest is that this network is built of corner sharing CoO 4 tetrahedra, as opposed to the more common octahedral or pyramidal arrangement of other frustrated magnetic oxides. The ground state of this new topology is yet unknown, although one would suspect that th...

SnS 2 has been extensive studied as an anode material for sodium storage owing to its high theoretical specific capacity, whereas the unsatisfied initial Coulombic efficiency (ICE) caused by the partial irreversible conversion reaction during the charge/discharge process is one of the critical issues that hamper its practical applications. Hence, heterostructured SnS 2 /Mn 2 SnS 4 /carbon nanoboxes (SMS/C NBs) have been developed by a facial wet-chemical method and utilized as the anode material of sodium ion batteries. SMS/C NBs can deliver an initial capacity of 841.2 mAh g −1 with high ICE of 90.8%, excellent rate capability (752.3, 604.7, 570.1, 546.9, 519.7, and 488.7 mAh g −1 at the current rate of 0.1, 0.5, 1.0, 2.0, 5.0, and 10.0 A g −1 , respectively), and long cycling stability (522.5 mAh g −1 at 5.0 A g −1 after 500 cycles). The existence of SnS 2 /Mn 2 SnS 4 heterojunctions can effectively stabilize the reaction products Sn and Na 2 S, greatly prevent the coarsening of nanosized Sn 0 , and enhance reversible conversion−alloying reaction, which play a key role in improving the ICE and extending the cycling performance. Moreover, the heterostructured SMS coupled with the interacting carbon network provides efficient channels for electrons and Na + diffusion, resulting in an excellent rate performance.

The quasi-2D nickelate La 4 Ni 3 O 8 (La-438), consisting of trilayer networks of square planar Ni ions, is a member of the so-called T′ family, which is derived from the Ruddlesden-Popper (R-P) parent compound La 4 Ni 3 O 10−x by removing two oxygen atoms and rearranging the rock salt layers to fluorite-type layers. Although previous studies on polycrystalline samples have identified a 105-K phase transition with a pronounced electronic and magnetic response but weak lattice character, no consensus on the origin of this transition has been reached. Here, we show using synchrotron X-ray diffraction on high-pO 2 floating zone-grown single crystals that this transition is associated with a real space ordering of charge into a quasi-2D charge stripe ground state. The charge stripe superlattice propagation vector, q = (2/3, 0, 1), corresponds with that found in the related 1/3-hole doped single-layer R-P nickelate, La 5/3 Sr 1/3 NiO 4 (LSNO-1/3; Ni 2.33+ ), with orientation at 45°to the Ni-O bonds. The charge stripes in La-438 are weakly correlated along c to form a staggered ABAB stacking that reduces the Coulomb repulsion among the stripes. Surprisingly, however, we find that the charge stripes within each trilayer of La-438 are stacked in phase from one layer to the next, at odds with any simple Coulomb repulsion argument.charge stripe | charge order | nickelate | strongly correlated materials | transition metal oxides C ompetition between localized and itinerant electron behavior is an organizing construct in our understanding of correlated electron transition metal oxide (TMO) physics (1-4). Some of the most compelling phenomenology in these materials occurs in the mixed valent state for the transition metal, which is set by composition, doping, and anion coordination of the metal. Many mixed valent TMOs adopt insulating "charge ordered" states, in which an inhomogeneous but long-range ordered configuration of the charge density condenses from a uniform metallic state (5). The real space pattern of charge order varies by material (6-9), but a typically observed motif is some variety of charge stripes. Such stripes have been observed in cobaltites (10-12), cuprates (13-15), nickelates (16)(17)(18)(19), and manganites (20-22), albeit with highly materials-dependent configurations that hinge on a balance among Coulomb, lattice, and magnetic exchange energies. For instance, charge stripes in layered nickelates typically stagger themselves from layer to layer to reduce the collective electrostatic energy arising from the charge disproportionation (9, 18).Indeed, the case of nickelates plays a prominent role in charge stripe physics (6-9, 16-19, 23-28), because mixed valent Ni 2+ (d 8 ) and Ni 3+ (d 7 ) compounds, such as La 2 − x Sr x NiO 4 (LSNO), are structurally and electronically related to high T c superconductors and thus, have been targeted as potential alternatives to the cuprates. Instead of superconductivity, however, the ground state of such quasi-2D, octahedrally coordinated nickelates is marked by stati...